Assay of Borax or Sodium tetraborate

Na2B4O7.10H2OM.wt.O M. wt. 381.4 ‰ Introduction

Borax (sodium tetraborate decahydrated) is soluble in 20 parts of water. It should be kept in well closed container. Borax is used as antiseptic and astringent in mouth paints. Assay of Borax

‰ Chemical principle

Borax is a derived from a weak acid and a strong base, so its aqueous solution can be assayed using a standard N/2 hydrochloric acid solution in an acid- base .

Borax samples may sometimes be contaminated with or . Thus two are carried out; ƒ the first is for borax (and sodium carbonate if present) with hydrochloric acid, and ƒ the second is for boric acid with . The corrected volumes of the two titrations can then tell the presence and type of impurities, besides the quantities of each. Assay of Borax

‰ Chemical principle

Reaction between borax and hydrochloric acid liberates boric acid

(H3BO3), a very weak acid that needs to increase its acidity (by converting it into a strong complex acid) so that it can react with sodium hydroxide by direct titration. This is achieved by the addition of excess of mannitol, a poly hydroxylated alcohol. MitlMannityl biboricacid complex is fdformed which isastrong acid. Assay of Borax

‰ Chemical principle

2 2

2 2

2 2

mannityl boric acid Assay of Borax

‰ Procedure

ƒ wash two burettes with D. W. and the titrant (HCl solution for the first one; and NaOH solution for the second one) ƒ fill the first one with N/2 HCl solution and the other with N/1 NaOH solution to a level (adjust them) ƒ put 10 mL of your unknown borax solution in a clean conical flask ƒ add 10 mL of D. W. ƒ add 2 drops of methyl red solution ƒ start the first titration by adding N/2 HCl drop wise with shaking until the colour chnages from (pH=6) to pink (pH=4.4) Assay of Borax

‰ Procedure

ƒ boil the titration liquid to expell the generated carbon dioxide (if any) and cool. If the colour changes back into yellow, resume titration with HCl until you get the pink colour ƒ record thevolume of HCl used Assay of Borax

‰ Procedure

ƒadd 4 gm of mannitol and 2 drops of phenolphthalein solution (pH=8.2, colourless; pH-10, pink) ƒ start the second titration by adding N/1 NaOH solution drop wise with shaking until the solution changes from pikink toyellow and then topikink again ƒrecord the volume of NaOH solution used Assay of Borax ‰ Titration apparutus Assay of Borax

‰ Discussion

The solution of the first titration should be boiled after the end point to expell carbon dioxide generated (if sodium carbonate is present):

This is to prevent the formation of carbonic acid which may affect the result of the second titration with sodium hydroxide solution.

Mannitol is added in excess quantity to prevent backword hydrolysis of mannityl boric acid complex. Assay of Borax

‰ Results and calculations

You must correct the volumes of both HCl and Note NaOH solutions into the specified normalites

V1 isthevolumeofN/2 HCl solution consumed (by borax and sodi um ccboearbonate if ppese)resent)

V2 is the volume of N/1 NaOH solution consumed (by liberated boric acid and contaminat boric acid if present) Assay of Borax

‰ Results and calculations

1. Both volumes of N/2HClsolutionconsumedandN/1 NaOH solution consumed are equal:

V1 = V2

then the sample of borax is pure.

Sometimes a difference of not more than 0.3 mL between the two volumes is allowed. In this case use the average.

Calculate the weight and percentage w/v of borax in your sample.

Each 0..09535 0 09535 gm of borax is equivalent to 11mLmLof N//22HClHClsolution Assay of Borax

‰ Results and calculations

2. ThevolumeofN/2HClsolutionconsumedismorethanthe volume of N/1 NaOH solution consumed :

V1 >V2 then the sample of borax is impure and contains, in addition to borax, sodium carbonate. V1 – V2 = the volume of N/2 HCl solution consumed by sodium carbonate ClCalcu late theweihights and percentages w/vof bhboth borax and sodium carbonate in your sample. Each 0..09535 0 09535 gmmofof borax is equivalent to 11mLmLof N//11NaOHNaOH solution Each 0..0265 0 0265gmgmgm of sodium carbonateis equivalent to 1mL 1 mLof N//22 HCl soltilution Assay of Borax

‰ Results and calculations

3. The volume of N/1 NaOH solution consumed is more than the volume of N/2 HCl solution consumed :

V2 >V1

mannitol 3 3 2 2 liberated and contaminant boric acid

then the sample of borax is impure and contains, in addition to borax, boric acid. Assay of Borax

‰ Results and calculations

V2 – V1 = the volume of N/1 NaOH solution consumed by the contaminat boric acid Calculate the weights and percentages w/v of both borax and contaminant boric acid in your sample.

Each 0..06184 0 06184gmgmgm of boric acid is equivalent to 1mL 1 mLof N//11NaOHNaOH solution

Each 0..09535 0 09535 gm of borax is equivalent to 11mLmLof N//22HClHClsolution